1. Technical Field
This invention relates to the art of AM radio broadcasting and, more particularly, to an RF power amplifier system having AC-DC power supplies using RF combining techniques.
2. Description of the Prior Art
The U.S. Patent to H. I. Swanson U.S. Pat. No. 4,580,111 discloses an amplitude modulator for use in AM radio broadcasting wherein the modulator serves to generate amplitude modulated signals by selectively turning on and off a plurality of RF amplifier modules to produce amplitude modulation. The amplifier modules include unit step modules that each, when on, provide an amplified voltage based on the operating voltage supplied to the amplifier. This operating voltage is a DC voltage obtained from rectifying a three phase AC input voltage. This operating voltage may be referred to as a B+ voltage.
It has been known in the prior art to provide B+ voltage as an operating voltage for amplifier modules by employing an AC to DC converter, referred to as an unregulated linear supply. Such a converter is illustrated herein at FIG. 2. This converter includes a transformer 100 having a delta connected primary winding network 102 connected to a three phase AC main line voltage supply, typically operating at 60 Hz. Two secondary winding arrangements 104 and 106 are provided. The secondary winding arrangements are respectively connected to fullwave rectifier assemblies 108 and 110 respectively. The rectifier assemblies 108 and 110 are frequently referred to as six-pulse rectifier assemblies. That is, in rectifying a three phase sinusoidal source six positive pulses are obtained for each cycle. These six positive pulses from the assemblies are combined with the use of an interphase transformer 112. The interphase transformer, together with a capacitor 114, provides the B+ operating voltage for a power amplifier module PAM-1 for supplying an amplified signal to a load L.
The limitations of the prior art in FIG. 2 include the requirement of step start contactors 120 and resistors 122 for each phase of the primary winding network 102 for on-off control and for in rush current limiting. Another limitation is the need for an interphase transformer 112 that combines the DC voltages from the two rectifier assemblies 108 and 110. The step start circuit adds extra expense to the system. The interphase transformer adds cost, reduces overall efficiency, and due to tolerance in the transformer and diodes, perfect balance of the transformer is not achievable. This results in an increase in the line current total harmonic distortion (THD) from a theoretical 7% up to approximately 12-15%. Combining DC supplies, such as that provided by rectifier assemblies 108 and 110, is difficult because any slight variation in voltage will cause a huge imbalance in current. The supply with the highest voltage will also supply the majority of the current. The interphase transformer (such as transformer 112 in FIG. 2) is the standard method of combining the output voltages obtained from two DC rectifier assemblies in a three phase system (to obtain a combined voltage having 12 pulses per cycle). The transformer works because the two rectifier assemblies are fed with AC voltages that are phased xc2x1/xc2x115 degrees of the main input phases. This allows each rectifier assembly to produce a DC voltage with a 360 Hz ripple. Since each rectifier assembly is out of phase from the other, the output voltages from the two rectifier assemblies can be combined by the interphase transformer creating a single DC voltage supply with a 720 Hz ripple. The interphase transformer acts as a balance. The rectifier assembly with the higher voltage will source the majority of the current, as the current in one side increases the effective impedance seen by the higher voltage rectifier assembly will increase causing a decrease in the effective output voltage and allowing the other rectifier assembly to start sharing the load.
The interphase transformer is constructed from copper and magnetic steel and is lossy, thereby lowering the overall system efficiency. Since the two rectifier assemblies are never going to be exactly alike (thus the need for the interphase transformer) the balance between the two halves will introduce line current harmonic distortions as the current balance in each rectifier assembly is increased. Theoretically, the ideal 12-pulse system should provide line current total harmonic distortion (THD) of 7%. As a practical matter this is never achieved and real world values of 15-20% are commonly seen. Line chokes and other methods can be employed to reduce the THD to 10% but these require extra components which add cost, lower efficiency, and are often large and bulky.
It is an object of the present invention to provide an AC-DC power supply with RF combining in such a manner that it eliminates the need for an interphase transformer and obviates the need for step start contactors and associated resistors.
In accordance with the present invention, an RF power amplifier system is provided that includes an RF source that provides an RF carrier signal having a given RF carrier frequency. An RF amplifier module amplifies the carrier signal and this module includes first and second RF amplifiers having a common input to receive a turn on signal that turns on both amplifiers. The outputs of the amplifiers are coupled together in parallel. A controller supplies a turn on signal to the common input to turn on the first and second amplifiers. A first AC to DC supply provides a first DC operating voltage obtained from a first multiphase AC supply and applies this to the amplifier module. The first DC voltage exhibits a ripple frequency dependent upon the number of phases in the first multiphase supply. A second AC to DC supply provides a second DC operating voltage from a second multiphase AC supply and applies this to the amplifier module. The second DC operating voltage exhibits a ripple frequency dependent upon the number of phases in the second multiphase supply.